It has been known for some time that certain hydroxy aromatic compounds are effective antioxidants useful in a wide range of applications. For example, the food additive commonly known as BHA is 2-t-butyl-4-methoxyphenol. Other phenols have been utilized as antioxidants in petroleum products, in plastics, in lubricants, and in other applications where increased oxidative stability is desired. Certain polyalkylated hydroxy-substituted aromatics are especially effective antioxidants. Thus, 2,4-di-t-butylphenol is of commercial utility as an antioxidant when added to fuel oils, such as gasoline. To utilize the antioxidant properties of such compounds it is incumbent to have a method of selectively alkylating hydroxy-substituted aromatic compounds in the position ortho to the hydroxy group.
The methods of alkylating hydroxy-substituted aromatic compounds are legion and well known to the skilled artisan in this field. Those methods based on strong acids, such as phosphoric and sulfuric acids, or strong Lewis acids, such as aluminum chloride, possess the disadvantage that considerable intralmolecular rearrangement, disporportionation, and transalkylation attend the desired alkylation. Thus, the final product using such catalysts tends to reflect thermodynamic control, i.e., given sufficient time an equilibrium mixture will result. For example, monoalkylation of p-cresol with introduction of the group R using the above catalysts may give a mixture of products according to the reaction. ##STR1## Since an object of the present invention is to provide compounds having at least one alkyl group ortho to the hydroxyl, and since such compounds generally are not thermodynamically more favored than other isomeric alkylated hydroxy-substituted aromatic compounds, the kinds of catalysts described above are unsuitable for efficient synthesis of these products. An additional disadvantage of these catalysts is that the reaction product is a complex mixture of isomers and homologs so that separation of pure components is a difficult if not a near-impossible task.
Use of weaker Lewis acids as catalysts alleviates the problem somewhat. Thus, in U.S. Pat. Nos. 3,290,389 and 3,367,981 are described processes in which alumina is used to alkylate phenols with preferential introduction of alkyl groups at the ortho position. However, because alumina is a relatively weak acid its catalytic activity is low relative to the stronger acids discussed above, necessitating minimum reaction temperatures of about 250.degree. C. and above for several hours to achieve polyalkylation. At these reaction conditions several undesirable side reactions may occur, such as oligomerization of the olefin used as the alkylating agent, thermal cracking to some of the reaction products, and significant disproportionation of some of the alkylated phenols thus formed. All these reactions are undesirable in the context of affording products containing relatively less of those phenols having the greatest antioxidant properties, in the context of affording complex mixtures from which separation of the most desirable component is difficult and tedious, and in the context that one of the reactants is consumed to give useless by-products. Nonetheless, under specified conditions alumina retains a place in the arsenal of available catalysts.